In many advanced telecommunication systems, particularly optical systems, Point of Use Power Supplies (PUPS) are used to supply conditioned power to each electronic card. Some of the basic requirements for this power conversion are:
(1) High power density PA0 (2) High efficiency PA0 (3) Low EMI (both conducted and radiated), and PA0 (4) Constant operating frequency PA0 (1) Full-Resonant DC/DC converters PA0 (2) Quasi-Resonant DC/DC converters PA0 (3) Multi-Resonant DC/DC converters
In order to achieve high power densities, there is a trend to operate power supplies at higher switching frequencies. As the switching frequencies increase, the switching losses associated with the turn-on and turn-off of the devices in the power supplies also increase. In switch mode power supplies, these losses are so significant that the operation of the power supplies at very high frequencies are prohibitive due to low conversion efficiencies. However, in resonant mode power supplies, the switching losses are low which allow the operation of resonant converters at very high frequencies. The resonant mode DC/DC converter can be classified into the following categories:
A number of U.S. Patents, e.g. U.S. Pat. No. 4,814,962, issued Mar. 21, 1989, to Magalhaes et al; U.S. Pat. No. 4,679,129, issued Jul. 7, 1989, to Sakakibara et al; U.S. Pat. No. 4,355,243, issued Oct. 19, 1982, to Tellert; U.S. Pat. No. 4,935,857, issued Jun. 19, 1990, to Nguyen et al; U.S. Pat. No. 4,992,919, issued Feb. 12, 1991, to Lee et al; and U.S. Pat. No. 4,833,584, issued May 23, 1989, to Divan, describe full-resonant DC/DC converters. U.S. Pat. Nos. 4,720,667 and 4,720,668, both issued to Lee et al on Jan. 19, 1988, on the other hand, disclose quasi-resonant DC/DC converters, while U.S. Pat. No. 4,841,220, issued Jun. 20, 1989, to Tabisz et al, and U.S. Pat. No. 4,857,822, issued Aug. 15, 1989, to Tabisz et al teach multi-resonant DC/DC converters. The converter topologies reported in the referenced patents have reduced to near zero switching losses, however, their output voltage is controlled and/or changed by varying the operating/switching frequency. These converters are, therefore, unsuitable for the application where system synchronization is required, e.g. telecommunications.
The present inventor's co-pending U.S. patent applications, Ser. Nos. 07/627,157 and 07/676,597, teach constant frequency resonant DC/DC converters. These converters exhibit near zero switching losses and can be operated at very high frequency. However, their power and control circuitry are complex and have a high component count. These converter circuits offer high power densities at medium-to-high power levels, however, at low-to-medium power levels, these converters do not give high power densities due to a high component count.
Canadian Patent No. 1,167,916, issued May 22, 1984, to White describes constant frequency half-bridge series resonant DC/DC converters. This converter circuit uses symmetrical pulse-width modulation (PWM) to vary the periodic energization of the series tuned circuit employed to drive a transformer and an associated rectifier. The symmetrical PWM series resonant circuit as taught in the patent has the following disadvantages:
(1) turn-on and turn-off losses are present for both the switches;
(2) high switching transient voltages are caused across the switches due to recovery current of the body diodes whenever the switch is turned-off after the conduction of its body diode.
(3) Any snubber capacitor (including drain-to-source capacitor) which is placed across the switch to absorb high switching transient voltages is discharged into the switch on turn-on. This gives rise to 1/2 CV.sup.2 losses.
The above losses increase with increasing switching frequency. This limits the operation of the converter at high frequencies.